1. Field of the Invention
The disclosed invention generally relates to the field of simulcast transmission systems, and more particularly a method and apparatus for automatically synchronizing the transmissions in a wide area simulcast transmission system.
2. Description of the Prior Art
A number of methods have been proposed or are in use today for automatically synchronizing the message transmissions of transmitters utilized in simulcast transmission systems. One such system is described in U.S. Pat. No. 4,718,109 to Breeden et al., entitled "Automatic Synchronization System" which is assigned to the Assignee of the present invention. A simulcast transmitter system is described wherein a master transmitter was centrally located within a plurality of secondary transmitters disposed in an annular fashion around the central transmitter. The innermost annular ring of transmitters were synchronized to the master transmitter, while the remainder of the system transmitters were disabled. The next adjacent annular band of transmitters were then synchronized to the innermost annular band and the process was repeated until every annular band in the system was synchronized. Such a synchronizing arrangement guaranteed adjacent annular bands were properly synchronized, however such a system did not necessarily provide for variations in delay which were introduced do to not utilizing a common signal source for making the delay measurements.
An alternate method of synchronizing the transmitters in a simulcast transmission system having a large number of transmitters is shown in FIG. 1. An important factor in determining the regularity to which the transmissions in such a simulcast transmission system was synchronized was the time required to complete the transmitter propagation delay measurement sequence. For a large simulcast transmission system, such as one having forty transmitters, delay measurement times of forty seconds and more were typical when each region was sequentially accessed for measuring the individual transmitter propagation delays. FIG. 1 shows a typical large multi-transmitter simulcast system 100 which has been divided into a plurality of smaller transmission regions 102, each transmission region 102 having a plurality of regional transmitters 104 responsive to a regional controller 106 for controlling the transmission of messages and further for controlling the transmission of information utilized for synchronization of the transmitter transmissions. Each transmission region 102 included one or more regional receivers 108 (only one of which is shown), which was coupled to the corresponding regional controller to provide monitoring of delay measurement signals required to enable the measurement of the inter-regional propagation delays for each of the regional transmitters in each transmission region 102. By splitting the simulcast transmitter system 100 into the smaller transmission regions 102, the inter-regional propagation delay measurements could be simultaneously measured for regional transmitters in alternate transmission regions, such as shown for regional transmitters 104 and 104" within transmission regions 102 and 102", respectively, thereby reducing the total time required to synchronize transmissions within the system. Measurement of the transmitter propagation delays as shown in FIG. 1, while speeding up the inter-regional propagation delay measurement process, presented a new set of problems, such as that of measuring the intra-regional propagation delays required to synchronize the transmitters in adjacent transmission regions.
In order to measure these intra-regional propagation delays, an output 110 of one of the regional controllers 106 was redirected to a regional transmitter 104 in an adjacent transmission region, as shown in FIG. 1, in order to establish a signal source for the intra-regional measurements. Once the intra-regional transmission propagation delays were measured, the intra-regional differential propagation delays were computed and then added to the inter-regional differential propagation delays for each transmission region to determine the total transmission delay required for each transmitter to synchronize the transmissions of the transmitters within each transmission region and between transmission regions.
A number of problems arose from the method of FIG. 1 for synchronizing the transmissions of such a simulcast transmitter transmission system. The intra-regional transmission propagation delays required a means for switching between two transmission sources for the same transmitter, i.e. controller 106 and controller 106'. This switching of sources added errors to the measurements consisting of delays introduced by the added signal path utilized to make the intra-regional measurements, which could easily approach hundreds of microseconds of added delay. When multiply adjacent transmissions regions occurred, i.e. where more than two transmission regions overlaped, additional switching hardware was required to interconnect each of the regions for measurement, further contributing to errors in the propagation delay measurements, and adding substantially to the cost of the system. The method of FIG. 1 also restricted cross check measurements between the adjacent regions without the utilization of additional hardware to provide such cross check measurements. The method of FIG. 1, also became inoperative in those instances when the transmitter used to compare intra-regional measurements became inoperative. To resolve this problem required additional hardware in the form of redundant switching to other transmitters within the transmission regions to be available when the primary transmitter failed. The method of FIG. 1 also precluded restructuring of the transmitters in the system as the system operator deemed appropriate, such as when a better combination of transmitters was determined to provide for more accurate propagation delay measurements within the simulcast transmitter system.